(a) Technical Field
Embodiments of the present disclosure are directed to an optical modulation device and a driving method thereof. More particularly, embodiments of the present disclosure are directed to an optical modulation device that includes a liquid crystal, a driving method thereof, and an optical device using the same.
(b) Discussion of the Related Art
Recently, optical devices that use an optical modulation device for modulating an optical characteristic have been developed. For example, optical display devices for displaying 3D images have garnered much attention, and they require an optical modulation device for separating images and transmitting the same at different times so that a viewer may perceive them as stereoscopic images. An optical modulation device may be used for a stereoscopic image display device without the need of viewing equipment, and includes a lens and a prism for changing the light path of an image from a display device, and transmitting it at a desired time.
Light diffraction by phase modulation can be used to change a direction of incident light.
When polarized light passes through an optical modulation device, such as a phase delay unit, its polarized state is changed. For example, when circularly polarized light is input to a half-wave plate, the rotation direction of the circularly polarized light is reversed, and the light is output. When left circularly polarized light passes through a half-wave plate, right circularly polarized light is output. In this case, the phase of the circularly polarized output light differs depending on an optical axis of the half-wave plate, that is, an angle of a slow axis. Specifically, when an optical axis of the half-wave plate rotates in-plane by φ, a phase of the output light changes by 2φ. Therefore, if the optical axis of the half-wave plate rotates by 180 degrees (π radians) in a spatial, x-axis direction, and phase modulated light can be output with a phase change of 360 degrees (2π radians) in the x-axis direction. When an optical modulation device changes the light phase from 0 to 2π according to position, a diffraction lattice or prism for changing or redirecting the direction of the passed light can be realized.
Liquid crystals can be used to control the optical axis of an optical modulation device, such as a half-wave plate, according to the position. Regarding an optical modulation device realized as a phase delay unit using liquid crystals, different phase modulations can be achieved as a function of position by rotating a long axis of the liquid crystal molecules by applying an electric field to a liquid crystal layer. The phase of light that propagates through and out of the optical modulation device can be determined by the direction of the liquid crystal molecules, that is, an azimuthal angle.
To perform consecutive phase modulations with an optical modulation device that uses liquid crystal to realize a prism, a diffraction lattice, or a lens, the liquid crystal molecules are arranged so that the long axis of the liquid crystal molecules may consecutively changed as a function of position. For the output light to have a phase profile that changes from 0 to 2π by position, the optical axis of the half-wave plate changes from 0 to π. For this purpose, a substrate that is adjacent to the liquid crystal layer may be aligned in different directions, which can complicate the process. When a precise alignment is needed, it may be challenging to achieve with a typical process, such as a rubbing process, which may cause display defects when used in the display device.